Since the beginning of the 1990s, incidents with engines and anomalies in aerodynamic speed and temperature measurements in airliners have been observed at high altitude and low temperature. In 2004, a working group on the harmonisation of engines known as EHWG (Engine Harmonisation Working Group) established that the icing conditions due to the presence of supercooled large droplets (SLDs), to the presence of ice crystals or to the simultaneous presence of SLDs and ice crystals gave rise to these anomalies. The American Federal Aviation Administration (FAA) and the European Aviation Safety Agency (EASA) introduced, in November 2014, changes to the regulations in force for overcoming the threat represented by such icy conditions for the safety of aircraft in flight. These new regulations are described in the document entitled “Aircraft and Engine Certification Requirements for Supercooled Large Drop, Mixed Phase and Ice Crystal Icing Conditions, Federal Aviation Administration Docket No. FAA-2010-0636, Amendment Nos. 25-140 and 33-34, Nov. 4, 2004”.
The regulation CS25 and in particular Appendix O thereof (“14 CFR part 25, Appendix O”) stipulates that water droplets with a diameter between 5 micrometres and 2300 micrometres must be able to be detected. The regulation CS33 and in particular Appendix D thereof (“14 CFR part 33, Appendix D”) stipulates that ice crystals with a size of between 5 micrometres and 2700 micrometres must be able to be detected.
The interferometric laser imagining technique for determining the size of droplets, generally referred to as the ILIDS technique (“Interferometric Laser Imaging for Droplet Sizing”), developed about 30 years ago, makes it possible to measure the size of spherical droplets in one plane (analysis in two dimensions). According to this technique, the water droplets are illuminated by a laser sheet polarised at a specific observation angle. Two light points, referred to as glare points, are visible on the surface of the droplets. These glare points create an interference pattern having the form of parallel fringes when they are imaged in a plane outside the focal plane of an optical system. The interfringe, that is to say the distance between two successive fringes, is inversely proportional to the distance between these two glare points. Thus an interfringe measurement makes it possible to determine the diameter of the droplets.
This technique has also been used to determine a size of a solid particle as described in the documents “Z. ULANOWSKI et al, Retrieving the size of particles with rough and complex surfaces from two-dimensional scattering patterns. Journal of Quantitative Spectroscopy and Radiative Transfer. 2012” and “D CHICEA. Biospeckle size and contrast measurement application in particle sizing and concentration assessment. Biophysics, Sep. 12, 2006”. According to these documents, when a solid ice crystal or any shape and with a rough surface is illuminated by a laser sheet, a multitude of glare points are visible on the surface of the crystal. The image outside the focal plane shows a speckle image. The size of the speckle grain is inversely proportional to the largest size of the visible ice crystal.
However, ILIDS out-of-focus interferometric imaging makes it possible to measure water droplets or ice crystals the sizes of which vary in a ratio of approximately a decade only, in particular because of the large size of water droplets and/or ice crystals and the limited performance of imagers existing on the market.
However, the size of the water drops and ice crystals present in the atmosphere varies between 5 μm and 2700 μm. This range of sizes is much greater than the range of sizes that can be determined by conventional interferometric imaging.